An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by a combination of both methods. When weight is applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
Different types of drill bits work more efficiently against different formation hardnesses. For example, drill bits containing cutting elements that are designed to shear the formation frequently drill formations that range from soft to medium hard. These cutting elements often have a working surface of polycrystalline diamond (PCD) and are often referred to as polycrystalline diamond compacts (PDCs). Drill bits containing PDCs as the cutting elements are often referred to as PDC drill bits.
Roller cone drill bits are efficient and effective for drilling through formation materials that are of medium to hard hardness. The mechanism for drilling with a roller cone drill bit is primarily a crushing and gouging action in which the cutting elements (e.g., inserts) of the rotating cones are impacted against the earthen formation material. This action compresses the material beyond its compressive strength and allows the drill bit to cut through the earthen formation.
For still harder formation materials, the mechanism for drilling changes from shearing to abrasion. For abrasive drilling, drill bits having fixed abrasive elements are preferred. While PDC drill bits are known to be effective in some formations, they have been found to be less effective for hard, very abrasive formations such as sandstone. For these hard formations, cutting structures that comprise abrasive particles, such as diamond grit, impregnated in a supporting matrix material are effective. In the discussion that follows, drill bits of this type are referred to as “impregnated” drill bits.
Impregnated drill bits are commonly used for boring holes in very hard or abrasive rock formations such as sandstone, quartz, basalt, granite, chert, and dolomite. Impregnated drill bits use a scouring or abrading-type of action to cut the earthen formation. The cutting face of such bits contains abrasive particles distributed within a supporting material to form an abrasive layer. During operation of the drill bit, abrasive particles within the abrasive layer are gradually exposed as the supporting material is worn away. The continuous exposure of new abrasive particles by wear of the supporting material on the cutting face is the fundamental functional principle for impregnated drill bits.
The construction of the abrasive layer is of importance to the performance of impregnated drill bits. The abrasive layer typically contains diamonds and/or other ultra hard particles distributed within a suitable supporting material. The supporting material must have specifically controlled physical and mechanical properties in order to expose the abrasive particles at the proper rate.
Metal matrix composites are commonly used for the supporting material because the specific properties can be controlled by modifying the processing or components. The metal matrix usually combines a hard particulate phase with a ductile metallic binder phase. The hard particles often include metal carbides (e.g., tungsten carbide), refractory materials, and/or ceramic materials. The metallic binder often includes copper or other non-ferrous alloys. Common powder metallurgical methods, such as hot-pressing, sintering, and infiltration are used to form the components of the supporting material into a metal matrix composite. Specific changes in the quantities of the components and the subsequent processing conditions allow control of the hardness, toughness, erosion and abrasion resistance, and other properties of the metal matrix composite.
Proper movement of fluid used to remove the earthen formation cuttings and cool the exposed abrasive particles is important for the proper function and performance of impregnated drill bits. The cutting face of an impregnated drill bit typically includes an arrangement of recessed fluid paths (also referred to as channels or waterways) intended to promote uniform flow from a central plenum to the periphery of the drill bit. The fluid paths usually divide the matrix material into distinct raised blades with abrasive particles exposed on the tops of the blades. The fluid provides cooling for the exposed abrasive particles and forms a slurry with the rock cuttings. The slurry must travel across the blades which contributes to the wear of the supporting material.
An example of a prior art diamond impregnated drill bit is shown in FIG. 1. The impregnated bit 10 includes a shank 24 and a crown 26. Shank 24 is typically formed of steel and includes a threaded pin 28 for attachment to a drill string (not shown). Crown 26 has a cutting face 29 and outer side surface (or gage) 30. Crown 26 typically includes cutting structures such as blades 40. The blades 40 are separated by channels 16 that enable drilling fluid to flow between and both clean and cool the blades 40. Suitably, formers are placed into a mold cavity during the manufacturing process so that the infiltrated impregnated crown 26 includes a plurality of recesses (may also be referred to as holes, pockets or sockets) 34 that are sized and shaped to receive a corresponding plurality of inserts (e.g., hot pressed diamond impregnated inserts) 36. Additional formers are typically included to form recesses for fluid nozzles (not shown). Once crown 26 is formed, inserts 36 are mounted in the recesses 34 and affixed by any suitable method, such as brazing, adhesion, mechanical means such as interference fit, or the like. Alternatively, the inserts 36 may be placed into the mold cavity instead of the formers and subsequently infiltrated.
Impregnated drill bits are typically made from a solid body of matrix material formed by any one of a number of powder metallurgy processes known in the art. During the powder metallurgy process, abrasive particles and a matrix powder are infiltrated with a molten metallic binder material. Upon cooling, the bit body includes the metallic binder material, hard particles, and the abrasive particles suspended both near and on the surface of the drill bit. One example process for making the impregnated matrix for bit bodies involves hand mixing of matrix powder with the abrasive particles and an organic binder to make a paste. The paste is then packed into the desired areas of a mold cavity. The shank of the bit is supported in its proper position in the mold cavity along with any other necessary formers, e.g., those used to form holes to receive fluid nozzles. The remainder of the mold cavity is filled with additional matrix powder and optionally abrasive particles. Finally, an infiltrant metal binder, typically a nickel brass copper based alloy, is placed on top of the charge of powder. The mold is then heated sufficiently to melt the infiltrant binder and held at an elevated temperature for a sufficient period of time to allow it to flow into and bind the powder matrix.
As discussed above, during drilling, the supporting material and the abrasive particles themselves are worn away, thereby exposing new abrasive particles. Therefore, there exists a desire to maximize blade height and to maintain the height for as long as possible in order to drill more of the formation before having to remove the drill bit from the borehole. The cost of drilling a wellbore is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the worn drill bit must be changed in order to reach the targeted formation. This is the case because each time the drill bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the wellbore, section by section. Once the drill string has been retrieved and the new drill bit installed, the bit must be lowered to the bottom of the wellbore on the drill string, which again must be constructed section by section. This process, known as a “trip” of the drill string, requires considerable time, effort and expense.